An air impact operated boring tool is a widely used device for boring underground. In many cases, such tools are used to bore a horizontal hole under a highway or other structure where conventional trench type construction cannot be performed.
The typical air impact tool has a cone shaped nose and a relatively uniform diameter body extending rearward of the nose. The conical nose penetrates the earth as an air operated impact hammer within the tool strikes an anvil at the front of the tool, driving the tool forward. The uniform diameter body is typically the portion of the tool which contains the air impact hammer. Tools of this type can be manufactured in different diameters to drill different size holes.
However, occasionally, it is desired to bore a larger diameter hole than the tool diameter, or to bore a hole in multiple passes with the diameter of the bore being increased at each pass. To accomplish this, expanders have been developed for mounting on a tool which increase the effective outer diameter of the tool. Such expanders can be a permanent addition to the tool, such as a bubble along the length of the outer diameter of the tool or removable additions to the tool. Prior known designs include an expander which slides on the nose of the tool and rests on the front taper or conical nose of the body. Another known design encloses the entire tool with a threaded connection in the middle.
In use, a tool of given diameter can effectively be enlarged by the use of such an expander to bore a larger diameter hole in one pass. Also, the tool can be used without the expander to drill a bore having a diameter corresponding to the diameter of the tool on the first pass of the tool; the expander can then be mounted on the tool and used to enlarge the bore in a second pass by the tool.
All of the existing expanders have deficiencies in one regard or the other. Some of these deficiencies include manufacturing costs, tool performance, non-reversibility, and reliability. A need therefore exists for improved expanders to address some of these deficiencies.